The proposed research will explore the mechanism by which nerve growth factor (NGF) suppresses neoplasia and blocks cell proliferation in certain meoplastic target cells. The model system to be studied is the PC12 rat pheochromocytoma cell line, which stops dividing and differentiates into neuron-like cells in response to NGF. PC12 cells have been used extensively for studies of the mechanism of neuronal differentiation, but have been underutilized for studies of regulation of cell proliferation. The relationship of differentiation-promoting pathways to cessation of cell proliferation will be investigated. Several possible antiproliferative mechanisms will be explored, including possible NGF-activated suppression of mitogenic signals from EGF and insulin via loss of EGF and insulin cell surface receptors. An underexplored property of PC12 cells is the reversibility of the non-dividing, differentiated phenotype; removal of NGF results in dedifferentiation and resumption of neoplastic proliferation. The temporal and mechanistic relationship of dedifferentiation to cell cycle reentry and to changes in certain cellular proteins which may affect proliferation will be explored. PC12 cells, although neoplastic, differ from many neoplastic cells in their ability to respond to a growth regulatory signal, NGF. The mechanistic differences between NGF-responsive and NGF-nonresponsive cells will be studied by the attempted selection of PC12 sublines with altered or absent responses to NGF. Sublines to be selected are: 1) Mutagen-induced NGF-resistant lines form early passage cultures. 2) Sublines selected for elevation of ornithine decarboxylase (ODC), to determine whether high ODC activity confers resistance to NGF's growth-regulatory activity. 3) Sublines "transformed" by the tumor virus SV40, to determine whether introduction of a well-studied transforming viral genome will confer resistance to NGF's growth-regulatory action. Comparison of the properties of these lines to wildtype PC12 cells may reveal clues to how cancer cells lose responsiveness to growth-regulatory signals.